Optical members have been used for a wide range of applications including cameras, telescopes, and so forth. Optical members can be broadly categorized into two groups: crystal optical members and glass optical members. Applications of crystal optical members vary depending on their crystal systems. Cubic crystals are used as lenses for imaging optical systems and so forth. The use of cubic crystals with high optical isotropy results in a reduction in, for example, birefringence due to optical anisotropy. Glass optical members are essentially optically isotropic and thus can be widely used, only considering stress-induced birefringence.
With respect to a glass composition, PTL 1 discloses a glass containing 50 mole percent to 77 mole percent aluminum oxide (Al2O3) and 27 mole percent to 50 mole percent rare-earth oxide (RE2O3). PTL 1 also discloses lutetium (Lu) as a rare-earth element (RE). Furthermore, PTL 1 discloses a bulk single phase glass that lies within the heptagonal region of the ternary composition diagram of a RE2O3—Al2O3—SiO2 system defined by seven points having mole percent compositions described below.
That is, the seven points are as follows:    (1% RE2O3, 59% Al2O3, 40% SiO2),    (1% RE2O3, 71% Al2O3, 28% SiO2),    (23% RE2O3, 77% Al2O3, 0% SiO2),    (50% RE2O3, 50% Al2O3, 0% SiO2),    (50% RE2O3, 0% Al2O3, 50% SiO2),    (33% RE2O3, 33.33% Al2O3, 33.33% SiO2), and    (16.67% RE2O3, 50% Al2O3, 33.33% SiO2).In the case where the units of the mole percent compositions of the points are converted into cation percent, the points are indicated by closed circles in the ternary composition diagram of FIG. 6.
PTL 1 also discloses a bulk single phase glass containing    (a) 1 mole percent to 50 mole percent RE2O3,    (b) 0 mole percent to 71 mole percent Al2O3,    (c) 0 mole percent to 35 mole percent SiO2, and    (d) 0 mole percent to 15 mole percent other oxides,in which the total concentration of RE2O3 and Al2O3 is at least 55 mole percent. Furthermore, lutetium (Lu) is disclosed as a rare-earth element (RE). In the case where the units of the mole percent compositions of the points are converted into cation percent, the points are indicated by closed rhombuses in the ternary composition diagram of FIG. 6.
PTL 1 discloses a glass according to the foregoing technique, the glass being usable in optical applications where uniform mechanical and thermal properties are required. PTL 1 also discloses a homogenous bulk single phase glass and a glass with a high refractive index according to the foregoing technique.
Higher integration of semiconductor integrated circuits has increasingly required ultrafine pattern formation. A reduction projection exposure apparatus (stepper) using a step-and-repeat process for transferring a fine pattern onto a wafer grows more sophisticated. The wavelength of light emitted from a light source for exposure is shifted to a short wavelength. An optical member that receives attention in optical members for that purpose is a cubic calcium fluoride single crystal having a high transmittance in the ultraviolet region. In recent years, attempts have been made to develop optical members containing elements, such as Lu, Al, and Mg, which has a higher refractive index than Si, in order to achieve a higher refractive index of an optical member to provide a higher resolution. For example, development of cubic crystals, such as lutetium aluminum garnet single crystal (LuAG, Lu3Al5O12), magnesium oxide single crystal (MgO), and magnesium spinel single crystal (MgAl2O4), have been actively conducted. In particular, the lutetium aluminum garnet single crystal has a high refractive index and thus is a promising candidate. For example, the lutetium aluminum garnet single crystal has a refractive index of 2.1 at a wavelength of 193 nm. Silica glass has a refractive index of 1.56 at a wavelength of 193 nm. The calcium fluoride single crystal has a refractive index of 1.50 at the wavelength of 193 nm.
Single crystal materials disadvantageously exhibit intrinsic birefringence in the ultraviolet region. MgO and MgAl2O4 have intrinsic birefringence values of 70 nm/cm (extrapolated value) and 52 nm/cm (extrapolated value), respectively, which are much larger than that (3.4 nm/cm) of CaF2 (for example, see NPL 1). Thus, the development of a material that does not have intrinsic birefringence is required.
PTL 2 discloses a glass composition for ultraviolet light, the glass composition containing Lu, Al, and O, in which the total proportion of Lu, Al, and O is 99.99% by weight or more, and the glass composition contains 24% to 33% Lu and 67% to 76% Al in terms of cation percent.
PTL 2 discloses a glass composition for ultraviolet light, the glass composition being suitable for a final lens of an immersion projection-type exposure apparatus, having a high refractive index and a high transmittance, and exhibiting low or no intrinsic birefringence (IBR) or low or no stress birefringence (SBR), and discloses an optical device using the glass composition.
PTL 1 discloses a homogeneous single phase glass having a high refractive index and being usable for optical applications. However, the glass has a low transmittance in the ultraviolet region. Furthermore, there is a problem in which a relatively stable, large-sized glass cannot be produced.
PTL 2 discloses a glass composition with a novel composition for ultraviolet light, the glass composition being suitable for a final lens of an immersion projection-type exposure apparatus, having a high refractive index and a high transmittance, and exhibiting low or no intrinsic birefringence and low or no stress birefringence. However, the glass-forming region is narrow, thereby failing to produce a relatively stable, large-sized glass.
In the case where a single crystal, such as LuAG, is used as an optical material, the single crystal exhibits intrinsic birefringence. Thus, the intrinsic birefringence needs to be compensated by, for example, a combination of lenses having different crystal orientations.